Method for controlling an optoelectronic component

ABSTRACT

A method for controlling an optoelectronic component that includes two waveguides. The refractive index of the first waveguide is changed periodically with a first control signal, the amplitude of which is changed between a first amplitude level and higher second amplitude level. The refractive index of the second waveguide is changed periodically with a second control signal, the amplitude of which is changed between the aforementioned first amplitude level and a lower third amplitude level. When the control signals are on their common first amplitude level, the refractive indices of the waveguides are equal and the phase difference between them is zero. When the first control signal is on the second amplitude level and the second control signal on the third amplitude level, the refractive indices of the waveguides are unequal so that their mutual phase difference has a predetermined target value.

FIELD OF THE INVENTION

The invention relates to optoelectronics and the subject of theinvention is the method described in the preamble of claim 1.

BACKGROUND OF THE INVENTION

In some optical applications, there is a need to change the phasedifference between two waveguides, such as waveguides on planarsubstrates or the like (e.g. optical fibers) as fast as possible. Thephase difference can be tuned by changing the optical length (lengthtimes the refractive index) in either one or both waveguides so that theoptical length difference between them changes.

For example, in an optical 2×2 switch based on a Mach-Zehnderinterferometer (MZI), as seen in FIG. 1, a phase difference changebetween two adjacent waveguides 3 and 4 in the area between directionalcouplers 16 and 17, induced by heating, makes the switch shift back andforth between the bar and cross states. In the bar state (on-state), theoptical power coming from one input is directed to the output of thesame waveguide, while in the cross state (off-state) the same power isdirected to the output of the adjacent waveguide. If both directionalcouplers 16 and 17 of the inputs and outputs of the switch are ideal50:50 power splitters, then the switch is in a cross state (off-state)when the phase difference is 0° (±N·360°) and in a bar state (on-state),when the phase difference is 180° (±N·360°). Between the bar and crossstates, the coupling state of the switch changes as a cosine function ofthe phase difference. If directional couplers are non-ideal 50:50 powersplitters, then at least one of the bar and cross states is onlypartial, in which case such a partial coupling state the output power ofneither waveguide is zero and the optical power is split to both outputsin a certain proportion. When the directional couplers 16 and 17 arelossless and mutually identical, the transmission T_(x) of a waveguidethat is crosswise with respect to the input is as a function of thephase difference Δφ according to${T_{x} = {\frac{1}{2}{{\sin^{2}( {\pi\quad{r/2}} )}\lbrack {1 + {\cos( {\Delta\quad\phi} )}} \rbrack}}},$where r is the length of the directional coupler in relation to itsideal length. The transmission of the bar state is T₁ ₁=1−T_(x). Byusing only one input or output port of the aforementioned switch, or byusing a symmetrical 1×1 Mach-Zehnder interferometer operating in thesame manner, one can realise e.g. a tunable attenuator or an on/offswitch. With similar structures one can also realise e.g. tunablewavelength filters.

FIG. 2 represents a schematic cross-section of a known switch, which wasalready illustrated in FIG. 1. In this example, waveguides 3 and 4 areso called silicon-on-insulator (SOI) waveguides. The switch includes asubstrate 12 made of silicon (Si) and which is, in this example, approx.0.5 mm thick. On top of the substrate 12 lies a thin SiO₂ layer 13,which is 1 μm thick. On top of the SiO₂ layer 13 is an approx. 5 μmthick silicon slab (Si) 14 covering the whole surrounding of thewaveguides. The waveguides 3, 4 are defined by local ridges. Along theridges the thickness of the silicon layer 14 is 10 μm. On top of thesilicon layer 14 is a 1 μm thick SiO₂ layer 15. Along the waveguides inpositions illustrated in FIG. 1, there are 0.5 μm thick heatingresistors 5 and 6 on top of the SiO₂ layer 15. The ridge acts as awaveguide and the field, illustrated by the dashed line, propagatesalong the ridge. Horizontally the waveguide is only bound by the steps,so that the silicon slab 14 extends all the way to the other waveguide.The light remains in the position of the ridge and propagates along theridge.

For controlling an optoelectronic component, two different methods arepreviously known. These methods are schematically illustrated in FIGS. 3and 4.

FIG. 3 represents a control signal amplitude of the switch as a functionof time, when only one of the two adjacent waveguides is modulated withthis first control signal, which is electric and substantiallyrectangular, and which produces a change in the refractive index (thatis, in optical length change) and, thus, produces a phase differencebetween the waveguides. The amplitude of the first control signal isrepresented in FIG. 3 by a thick black line. The phase differenceinduced between the waveguides is represented by a dotted line. In theoff-state of the switch the amplitude of the first control signal iszero and in the on-state it is in such a value that the optical lengthof the modulated waveguide has become shorter or longer by half awavelength (phase difference 180°).

FIG. 4 illustrates another previously known method, which is analternative to the method illustrated in FIG. 3, where one of thewaveguides is being modulated with an electric control signal thatsubstantially consists of two rectangular parts. In this method, thephase difference between the two waveguides can be raised from zero tothe desired target value faster than in the previously described firstknown method. The higher and substantially rectangular first part of thecontrol signal induces a very fast temperature rise in one of thewaveguides, because its peak amplitude is significantly higher than whatis needed to maintain the waveguide in its target temperature. Comparedto the first known method, this method consumes more power, but it hasthe advantage of faster rise time.

The methods represented by FIGS. 3 and 4 represent the technology whichis closest to the invention and correspond to the preambles of claims 1and 6. According to the known methods, the refractive index of the firstwaveguide is changed periodically with the first control signal, theamplitude of which is changed periodically between a first amplitudelevel I, which is substantially zero, and a second amplitude level II,which is higher than the first amplitude level. In the beginning of therise time period the amplitude of the first control signal can go to afourth amplitude level (IV), which is distinctly higher than the secondamplitude level. When the first control signal is on the first amplitudelevel I, which is substantially zero, the refractive indices of thefirst and second waveguide are equal and the phase difference betweenthem is zero. When the first control signal is on the second amplitudelevel II, the refractive indices of the first and second waveguide areunequal so that the phase difference is in a predetermined target value.In the rise time period of the phase difference the first amplitudelevel I forms a start level for the first control signal and the secondamplitude level II forms its target level. Similarly, in the fall timeperiod of the phase difference the second amplitude level II forms astart level for the first control signal and the first amplitude level Iforms its target level.

Furthermore, it is known that the optical length of a waveguide (and thephase of the light propagating along the waveguide) can be changed e.g.by heating, stressing or bending the waveguide, by producing an electricfield into the waveguide or by injecting current through the waveguide.Different modulation mechanisms have their advantages and disadvantageswith respect to e.g. speed, optical attenuation, electric powerconsumption, necessary modulation length and costs.

Known thermo-optical switches usually operate with frequencies reachingup to 1 kHz, at the most, but they are relatively simple and inexpensiveto fabricate. Their modulation speed is limited by the heat conductionfrom the heating resistor to the waveguide core and onwards away fromthe core, as well as by the heat capacity of the waveguide. In general,heating is more efficient and faster when the volume to be heated issmaller. Good thermal conductivity away from the waveguide, e.g. to anunderlying cooled substrate, makes the modulation faster, but it alsoincreases the electric power consumption. If a waveguide is small andefficiently thermally insulated from the surrounding, it can heat upfast but cool down slowly. In general, thermo-optical switches heat upsignificantly faster than they cool down. However, asilicon-on-insulator (SOI) waveguide represented in FIG. 2, for example,heats up and cools down almost as fast, because the heat efficientlyspreads along the horizontal silicon and then conducts from a broad areathrough the thin oxide layer into the silicon substrate. Experimentshave shown that due to the good thermal conductivity in SOI-waveguides,the back and forth 180° phase difference changes can be obtained withfrequencies reaching up to 10 kHz, which is somewhat faster than in thecommercial thermo-optic switches. While modulating one waveguide, theheating power is then approx. 0.3-0.4 W in the on-state and 0 W in theoff-state, which is still quite reasonable. The temperature of thewaveguide stabilises exponentially, so that, for example, a 90%modulation can be obtained much faster than a 99% modulation.

Furthermore, it is known that a control signal can be used to create anelectric field into the waveguide or to produce an electrical currentthrough the waveguide, which enables the realisation of significantlyfaster switches, but also these have some typical disadvantages, such ashigher optical attenuation and higher costs of the technology. Thesemethods also have a finite delay that limits the modulation speed.

Publication U.S. Pat. No. 5,173,956 describes an optical switch in whichthe refractive index is controlled by injecting electric current throughthe material for obtaining internal heating. The publication mentionsthat the associated switch can reach 1 MHz switching speed. As mentionedabove, current injection has the disadvantage of increased opticalattenuation.

Publication U.S. Pat. No. 6,278,822 involves an apparatus where thereare different materials between two waveguides and a current injectedthrough the materials simultaneously heats up one waveguide and coolsdown the other waveguide by exploiting the Peltier effect. When appliedto an optical switch, this solution can reach 10 MHz switching speed.The disadvantage of this method is that it requires significant changesin the switching structure and cannot, therefore, improve the switchingspeeds of existing switches.

Publication U.S. Pat. No. 6,351,578 describes an optical switch wherethe refractive index is changed by heating it with a first controlsignal that is illustrated in FIG. 4. The operation of the associatedswitch is not particularly sensitive to the exact values of the controlsignal amplitudes, because the refractive index change of the switchonly needs to exceed a given threshold value for deflecting light out ofthe waveguide. The method only reduces the rise time of the switch andit has not been applied for changing the phase difference between twoadjacent waveguides or for accurate tuning of the refractive index.

PURPOSE OF THE INVENTION

The purpose of the invention is to provide a method for changing thephase difference between two waveguides, such as waveguides on planarsubstrates or the like, significantly faster than the known methods, andwithout the need for any structural changes in the component. When theoptoelectronic component is, for example, an optical switch, itsswitching speed can be substantially increased.

SUMMARY OF THE INVENTION

The method of the invention is characterised in what is disclosed inclaims 1 and 6.

According to the invention, for changing the phase difference betweenthe waveguides fast from one desired value to another, the firstwaveguide is heated with a first control signal and the second waveguideis heated with a second control signal, so that during this phasedifference change, namely the rise or fall time period, both controlsignals are changed between their start and target levels so that thephase difference change is obtained significantly faster than what ispossible by using only one (first) control signal and already before therefractive indices of the waveguides are stabilised.

According to the invention, both waveguides are heated with the sameheating power during an off-state so that the first and second controlsignals are on a common first amplitude level (I), which is higher thanzero. Then the phase difference between waveguides is zero. Similarly,during an on-state the first waveguide is heated with a higher heatingpower, corresponding to a second amplitude level (II), and the secondwaveguide is heated with a lower heating power, corresponding to a thirdamplitude level (III). The third amplitude level can be zero, but in oneembodiment of the invention, it can also be higher than zero.

According to the invention, for reducing the phase difference rise timeperiod, the amplitude of the first control signal is adjusted from itsstart level, namely the first amplitude level (I), to a fourth amplitudelevel (IV), which is higher than its target level, before it is adjustedto its target level, namely the second amplitude level (II), andsimultaneously the amplitude of the second control signal is loweredfrom its start level, namely the first amplitude level (I), to itstarget level, namely the third amplitude level (III). Then the fasttemperature rise of the first waveguide is combined with the temperaturefall of the second waveguide, which leads to a faster phase differencechange than what can be reached by using only one control signal.Additionally, the first waveguide control signal is used to compensatefor the slow final cooling of the second waveguide so that during thefinal part of the rise time period both waveguides still slowly cooldown towards their own target temperatures, while their mutual phasedifference has already reached its target value and settled to it.

According to the invention, for reducing the phase difference fall time,the amplitude of the first control signal is lowered from its startlevel, namely the second amplitude level (II), to an eighth amplitudelevel (VIII), which is lower than its target level, before it isadjusted to its target level, namely the first amplitude level (I), andsimultaneously the amplitude of the second control signal is adjustedfrom its start level, namely the third amplitude level (III), to a ninthamplitude level (IX), which is higher than its target level, before itis adjusted to its target level, namely the first amplitude level (I).Then the temperature fall of the first waveguide is combined with arapid temperature rise of the second waveguide, which leads to a fasterphase difference change than what can be reached by using only onecontrol signal. Additionally, the second control signal is adjusted sothat at the last part of the fall time period the first and secondwaveguide cool down together so that both waveguides still slowly cooldown towards their common target temperature, while their mutual phasedifference has already reached its target value, namely zero, andsettled to it.

The method according to the invention has the advantage that it can beused to change the phase difference between two waveguides, such aswaveguides on planar substrates or the like, significantly faster thanwhat is possible by using known methods, and this can be achieved simplyby using appropriate control signal modulation without any structuralchanges in the component. Thus, the operation of existing optoelectroniccomponents can be enhanced simply by modifying their electric controlsignals to operate according to the invention. The amplitudes of thecontrol signals are not simply set to their desired target levels, as ina traditional manner (see FIG. 3), but the phase difference changes(rise and fall time periods) are implemented with fast control signalsthat vary as their power is concerned. Additionally, according to theinvention, the phase modulation efficiently exploits the temperaturechanges that appear simultaneously in both waveguides, and that togetheraccelerate phase difference changes and enable the phase difference tosettle to its target level already clearly before the waveguidetemperatures are settled. Then the phase difference can be changed toits desired value as fast as possible and the phase difference changecan be made as step-wise as possible. In one embodiment of the method,for reducing the phase difference rise time the fourth amplitude levelof the first control signal is chosen so high that the phase differenceclearly tends to rise above its predetermined target value, leading to aso-called overshoot of the refractive index. For compensating theovershoot the amplitude of the second control signal is raised to asixth amplitude level, which is higher than the fifth amplitude level.

In one embodiment of the method, for reducing the phase difference risetime the amplitude of the first control signal is set to a seventhamplitude level (VII), which is lower than the second amplitude level,before it is set to its target value, namely the second amplitude level.

In one embodiment of the method, for reducing the phase difference falltime the amplitude of the second control signal is lowered from theninth amplitude level to a tenth amplitude level, which is lower thanthe first amplitude, before it is set to its target level, namely thefirst amplitude level.

In one embodiment of the method, for reducing the phase difference falltime, the tenth amplitude level is chosen to be substantially equal tothe eighth amplitude level.

In one embodiment of the method, the third amplitude level, fifthamplitude level, seventh amplitude level, eighth amplitude level and/ortenth amplitude level are chosen to be zero.

In one embodiment of the method, the third amplitude level is chosen tobe higher than zero. In particular, this can be used to furtheraccelerate the cooling of the second waveguide to its target level atthe last part of the rise time period.

In one embodiment of the method, the phase difference between the firstand second waveguide is modulated with at least two or more successivephase modulators.

In one embodiment of the method, the successive modulators are guided tooperate in turn.

In one embodiment of the method, only some of the intended successivephase difference changes are implemented with the control signals of agiven first modulator. Other phase difference changes are implementedwith the control signal(s) of another modulator(s), which lie(s)successively with respect to the first modulator, so that the phasedifference changes induced by the successive modulators sum up. Then thenext phase difference change can be implemented with the next modulatorimmediately after the implementation of the previous phase differencechange, although the refractive indices of the waveguides associatedwith the previous modulator are not yet settled to their target levels.

In one embodiment of the method, at least two successive modulators aremutually different, so that the first modulator is significantly fasterand consumes more power than the second modulator, so that the firstmodulator is used for implementing fast and/or successive phasedifference changes and the second modulator is used for implementingslow and/or single phase difference changes as well as long staticoperating states, so that the average power consumption is significantlysmaller than what can be obtained by using only the first modulator, andthe maximum modulation speed is significantly higher than what can beobtained by using only the second modulator.

In one embodiment of the method, the control signals are optimisedduring the rise and/or fall time periods so that they depend, not onlyon the start and target levels of the given intended phase difference,but also on at least one phase difference level that precedes the startlevel and/or succeeds the target state of the phase difference, so thatthe optimisation takes into account such a potential settling time thatimmediately precedes and/or succeeds the given phase difference change,and during which settling time the phase difference has already reachedits target value, while the refractive indices of the waveguides havenot yet settled.

In one embodiment of the method, the target value of the phasedifference is set to approx. 180°.

In one embodiment of the method, the waveguides are realised on planarsubstrates.

In one embodiment of the method, the waveguides are chosen to be one ofthe following: SOI (silicon-on-insulator) waveguides, glass waveguides,polymer waveguides, compound semiconductor waveguides.

In one embodiment of the invention, the optoelectronic component ischosen to be an optical switch, such as an interferometric switch.

In one embodiment of the method, the component includes at least oneMach-Zehnder interferometer, and it forms an optical switch or a filter.

In one embodiment of the method, the optical switch is chosen to be athermo-optic switch, where the modulators are heating elements that heatup the waveguides and the electric control signals consist of controlvoltage/current pulses that are directed to the heating elements, sothat the amplitude level of control signals corresponds to the heatingpower induced in the heating elements by the control voltage/currentpulses.

BRIEF DESCRIPTION OF DRAWINGS

In the following, the invention is explained in detail with thedescription of embodiments and with reference to the accompanyingdrawings, where

FIG. 1 represents a previously known thermo-optic 2×2 switch based on aMach-Zehnder interferometer as seen from the top and as appropriatelymagnified.

FIG. 2 represents a magnified cross-section II-II from FIG. 1,

FIG. 3 schematically represents one known method for modulating thephase difference between two waveguides, namely the amplitude of acontrol signal as a function of time, when only one of the two adjacentwaveguides is modulated with an electric control signal, which has asubstantially rectangular shape,

FIG. 4 schematically represents another known method for modulating thephase difference between two waveguides, namely the amplitude of acontrol signal as a function of time, when only one of the two adjacentwaveguides is modulated with an electric control signal, whichsubstantially consists of two rectangular parts,

FIG. 5 schematically illustrates the amplitudes of control signals (e.g.heating powers or control voltages) as a function of time during therise (off→on) and fall time periods (on→off) between on and off states,according to the method of the invention,

FIG. 6 schematically illustrates the refractive indices (orcorresponding temperatures that have a direct effect on the refractiveindices) that are obtained with control signals, similar to those inFIG. 5, as a function of time during the rise and fall time periods,when it has been assumed that there is no significant delay between theelectrical and optical signals,

FIG. 7 schematically illustrates the phase difference between the twowaveguides as a function of time during the rise and fall time periods,corresponding to FIGS. 5 and 6,

FIG. 8 represents experimental results obtained, with the method of theinvention, from a thermo-optical switch at a frequency of 24,3 kHz, theFigure showing the measured output power P_(opt) of the optical signal,the amplitude of the first waveguide's control signal, namely theheating power P₁, and the amplitude of the second waveguide's controlsignal, namely the heating power P₂, as a function of time, and

FIG. 9 represents experimental results obtained, with the method of theinvention, from a thermo-optical switch at a frequency of 161 kHz, theFigure showing the measured output power P_(opt) of the optical signal,the amplitude of the first waveguide's control signal, namely theheating power P₁, and the amplitude of the second waveguide's controlsignal, namely the heating power P₂, as a function of time.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention is tested by applying thermo-opticmodulation to a 2×2 MZI-switch based on SOI waveguides. Such a knownswitch is presented in FIGS. 1 and 2. In the following, the modulationprinciple is mostly described in the context of this particularapplication. The phase difference modulation method of the inventioncan, however, be also applied in the context of many other waveguidestructures (such as glass and compound semiconductor waveguides),optical components (e.g. 1×1 switches/modulators and filters) andmodulation mechanisms (e.g. electro-optic).

As already noted above, in traditional phase difference modulation onlythe first control signal, or a corresponding control voltage (orcurrent), is changed, as illustrated in FIGS. 3 and 4. A change in thecontrol signal amplitude induces a change in the optical lengthdifference (and phase difference) from one value to another, while thisresulting change stabilises exponentially.

An example of the modulation principle of the invention is schematicallypresented in FIGS. 5-7. In FIG. 5 thick black line A represents changesin the first control signal 1 that controls the refractive index of thefirst waveguide 3 between different amplitude levels as a function oftime. FIG. 5 shows as if the setting of the control signal amplitudes todifferent amplitude levels would take place in a rectangular andstep-wise manner, which represents a certain kind of theoretically idealcontrol. As can be seen from FIGS. 8 and 9, which represent the realexperimental work, the control signal pulses are, in practise, somewhatrounded and, furthermore, the control signals can also intentionallyconsist of oblique or curved parts. Therefore, it is clear to a anyexpert in the field that the principle of the invention is not limitedto any particular shapes of the control signal, such as rectangularpulses. Dashed line B represents the changes in the second controlsignal 2 that controls the refractive index of the second waveguide 4between different amplitude levels as a function of time. FIG. 6illustrates the changes of the refractive indices in the first andsecond waveguides as a function of time. Curve C represents refractiveindex changes in the first waveguide 3, while curve D representsrefractive index changes in the second waveguide 4. Temperature changesin the waveguides behave similarly with the refractive index changes.FIG. 7 represents changes of the phase difference between the first andsecond waveguide 3 and 4 as a function of time.

During switching periods the objective is to change the phase differencebetween the waveguides as fast as possible to the desired value byadjusting the optical lengths of both waveguides separately and byshaping the control signals within the switching periods (rise andfall). According to the invention, both rise and fall time period isaccelerated with respect to the control methods illustrated in FIGS. 3and 4. So called off-state biasing (or preheating) is used, so thatduring an off-state both waveguides are being heated and the heatingpower levels (or amplitudes) of control signals 1 and 2 are set to thefirst amplitude level I.

From FIGS. 5-7 it can be seen that the rise time is being accelerated byraising the heating power of the first waveguide first as high aspossible and by simultaneously dropping the heating power of the secondwaveguide to zero. When approaching the target phase difference valuethe heating power of the first waveguide is lowered. For producing amaximally step-wise phase difference change the heating power of thefirst waveguide can be temporarily dropped to zero after the high peak.When the target phase difference has been reached the heating power ofthe first waveguide is adjusted so that phase difference between thewave guides remains at its target value while both waveguides cool downtowards their target temperatures. At the end of the rise time periodthe heating power of the first waveguide is set to its target value forthe on-state and the heating power of the second waveguide is set eitherto zero or to a small so called on-state bias value. In some cases therise time can be further shortened by rising the temperature of thefirst waveguide rapidly so high that the phase difference tends to passits target level. This overshoot can be compensated by directing aheating pulse (or pulses) to the other waveguide at an appropriatemoment. Then the phase difference reaches its target level as fast aspossible, and then both waveguides cool down towards their own targettemperatures for the on-state, while maintaining a constant phasedifference. Cooling down to the ambient temperature follows anexponential stabilisation curve, so a small on-state heating bias in thesecond waveguide allows a faster attainment of the equilibrium. Theheating bias is naturally not set on until the corresponding targettemperature is reached. Then the temperature of the second waveguidecools down freely (without heating) towards a virtual target level,namely the temperature that is reached when the second waveguide is notheated for a very long time, until the temperature enters the vicinityof its real target level. Then the second waveguide is again heated sothat its temperature settles to its real target level as fast aspossible. This avoids the very slow last part of the exponentialtemperature stabilisation.

According to a more general explanation, with reference to FIG. 5,during the rise time period the amplitude of the first control signal 1is raised from its off-state start level, namely the first amplitudelevel I, which is different than zero, to a fourth amplitude level IV,which is substantially higher than the final on-state target level,namely the second amplitude level II. Simultaneously, the amplitude ofthe second control signal 2 is lowered from its off-state start level,namely the first amplitude level I, to a fifth amplitude level V, whichis zero in this example. From FIG. 6 it can be seen that in theoff-state the refractive indices have a common and constant value, butduring the rise time period the refractive index C of the firstwaveguide 3 is increasing and the refractive index D of the secondwaveguide 4 is decreasing, in which case the phase difference betweenthe waveguides is increasing. The fourth amplitude level IV of the firstcontrol signal 1 can be set to be so high that the phase differenceclearly tends to rise above the predetermined phase difference targetvalue, which leads to a so-called overshoot, as is pointed out with anarrow in the refractive index curve C of the first waveguide in FIG. 6.For compensating the overshoot, the amplitude of the second controlsignal 2 can be raised to a sixth amplitude level VI, which is higherthan the target level for the amplitude of the second control signal 2,namely the third amplitude level III. On the other hand, although theovershoot of the first waveguide would not be compensated by the secondcontrol signal 2, a small overshoot in the first waveguide's refractiveindex can be used to compensate for the slow exponential stabilisationof the second waveguide's refractive index at the last part of the risetime period (not illustrated in FIGS. 5-7). The amplitude of the firstcontrol signal I is set to a seventh amplitude level VII, namely tozero, before it is set to its target value, namely the second amplitudelevel II. The compensation is pointed out with an arrow in therefractive index curve D of the second waveguide. At the end of the risetime period the amplitude of the first control signal 1 is set to itstarget level, namely the second amplitude level II, and the amplitude ofthe second control signal 2 is set to its on-state target level, namelythe third amplitude level III. From FIG. 6 it can be seen that in theon-state the refractive index of the first waveguide 1 (curve C) has itshigher constant value and the refractive index of the second waveguide 2(curve D) has its lower constant value, so that the phase differencebetween them is constant, as is presented in FIG. 7 by curve E whichrepresents the phase difference.

In a similar manner, the fall time of the phase difference can bereduced by directing a heating pulse to the second (colder) waveguide atthe same as the heating of the first waveguide is stopped so that it cancool down towards its constant off-state value. The objective is toraise the temperature of the second waveguide to a common value with thefirst waveguide's temperature as fast as possible, and then to let themcool down together towards their common and constant off-statetemperature (see FIG. 6). According to the invention, the off-statebiasing significantly accelerates the settling of the temperatures.Again, the heating bias is not turned on until the constant targettemperatures are reached.

According to a more general explanation, with reference to FIG. 5,during the fall time period of the phase difference, the amplitude ofthe first control signal 1 is lowered from its on-state start level,namely the second amplitude level II, to an eight amplitude level VIII,which in this case zero. Simultaneously, the amplitude of the secondcontrol signal 2 is raised from its start level, namely the thirdamplitude level III, to a ninth amplitude level IX, which issubstantially higher than the target level, namely the first amplitudelevel I. Before the amplitude of the second control signal 2 is set toits off-state target level, namely the first amplitude level I, theamplitude of the second control signal 2 is lowered from the ninthamplitude level IX to a tenth amplitude level X, which is in this casezero. Then the amplitude of the first control signal 1 and the secondcontrol signal 2 are set to their common target level, namely the firstamplitude level I. The resulting effects in the refractive index changescan be seen from FIG. 6. In the beginning of the fall time period therefractive index C of the first waveguide 3 starts to drop from itshigher constant on-state value and simultaneously the refractive index Dof the second waveguide 4 starts to rise from its lower on-stateconstant value. When the curves C and D meet, the refractive indices areequal and the phase difference is zero, as can be seen from the phasedifference curve E in FIG. 7. The waveguides cool down equally fast, sothat their refractive indices also decrease equally fast towards theoff-state constant value.

With respect to known methods, the modulation method of the inventionenables significantly shorter rise and fall times for the phasedifference changes. The phase difference reaches its target values veryfast and by using appropriate heating biases the waveguide temperaturesstabilise to their constant values almost as fast. If there is notenough time for the temperatures to settle between the rise and falltimes, then the temperature of one or both waveguides can start toslowly drift to a harmfully high level (see FIGS. 6 and 7). If thesettling of the temperatures limits the frequency of the phasedifference modulation clearly more than the actual rise and fall timesof the phase difference, then the frequency cannot be raised to thelimits posed by the rise and fall times in a straight-forward manner. Onthe other hand, in such a case the pulse shapes are clearly better,namely more rectangular, with respect to traditional modulation. This,on the other hand, makes it feasible to place at least two or more phasedifference modulators successively. Then they can operate in turn and,thus, enable double or multiple modulation frequency, depending on thenumber of modulators. For example, in the thermo-optic switchillustrated in FIG. 1 there are two successive phase differencemodulators 10 and 11, which both include a pair of adjacent heatingelements 5, 6 and 7, 8 for waveguides 3 and 4. The switch is operatedbetween the on- and off-states by using modulators in turn so that thefirst has the time to cool down while the second is being operated.

Any desired number of modulators can be arranged successively and theycan be either identical or dissimilar. The modulators can be operated inturn. It is possible to realise only some of intended successive phasedifference changes with control signals associated with one modulatorand to realise the other phase difference changes with control signalsassociated with one or several other successively arranged modulators sothat the phase difference changes caused by the different modulators sumup, in which case the next phase difference change can be implementedwith the next modulator as soon as the previous phase difference changeis obtained, although the refractive indices of the waveguides in theprevious modulator have not yet settled to their target levels.

If, for example, the thermal insulation of the waveguides is improved,then the fall time of a waveguide's temperature change can becomesignificantly longer than the rise time. A well insulated silicon corenaturally heats up faster than it cools down. By implementing theoptimisation of the optical signal's fall time according to theinvention one can make both the optical rise and fall time very short,but the stabilisation of the waveguide temperatures to valuescorresponding to static on/off-states may take a long time after thefall time. This settling time limits the continuous operating frequencybecause after a fast fall it is still necessary to have a long waitbefore the next rise can be implemented. If this wait is skipped, then acontinuous back and forth operation rises the waveguide temperatureshigher and higher, until the switch no longer operates in a desiredmanner. If the temperatures are given to settle, then the optical pulses(output signals) are still very rectangular at the maximum operatingfrequency. In traditional modulation, the rising of the frequencyusually makes the pulses more round and reduces the range of signalvariation until at a certain maximum frequency the pulses are so poorthat the frequency can no longer be raised. The rectangularcharacteristic of the pulses at the maximum frequency can be exploitedso that two or several modulators are arranged successively. In the caseof two modulators, for example, they can implement the intended phasedifference changes in turn and, thus, double the maximum operatingfrequency. The maximum number of successive modulators depends on therectangular characteristic of the optical response of one modulator(rise-, fall- and settling times) and a higher number of modulatorswould make the signal worse than allowed (cf. traditional modulation).

Long-term power consumption can be reduced and phase difference changescan be accelerated in case of need, when at least two successivemodulators are mutually different so that the first modulator issignificantly faster and consumes more power than the second modulator,so that the first modulator is used for implementing fast and/orsuccessive phase difference changes, and the second modulator is usedfor implementing slow and/or single phase difference changes and longstatic operating states, so that the average power consumption issignificantly smaller than what can be obtained by using only the firstmodulator, and the maximum modulation speed is significantly higher thanwhat can be obtained by using only the second modulator. When theoptical signal is to be held constant for longer periods of time, onlythe slow and low-power modulator is used to maintain the desired phasedifference. If the rise and/or fall time of the slow modulator issignificantly shorter than the settling time, then this low-powermodulator can also be used to implement single phase changes, and onlyseveral successive changes are implemented with the high-powermodulators. This way the long-term average power consumption can be verysmall, although the modulator can also produce very fast and continuousmodulation (with temporarily higher power consumption) when needed.

Transitions between the use of fast and slow modulators can be realisedeither abruptly or very slowly. In both cases, the phase differencechange induced by a modulator is, for example, 0→180° for one modulatorand 180→0° for the second, so that the total phase difference staysconstant. During an abrupt transition the optical signal may temporarilyweaken, but this is not harmful if it is combined with a simultaneouschange of the total phase difference.

Changes of the phase difference can be further optimised so that thecontrol signals are optimised during the rise and/or fall time periodsso that they depend, not only on the start and target levels of thegiven intended phase difference, but also on at least one phasedifference level that precedes the start level and/or succeeds thetarget level of the phase difference, so that the optimisation takesinto account such potential settling time(s) that immediately precedeand/or succeed the given phase difference change, and during whichsettling time(s) the phase difference has already reached its targetvalue, while the refractive indices of the waveguides have not yetsettled.

For example, in a thermo-optic component the heating signals between twobits can be controlled with four alternative ways depending on thedifferent bit sequences (00,11,01,10). The alternatives can be named asfollows: off (0→0), on (1→1), rise (0→1) and fall (1→0). The heatingperiod corresponding to a bit also depends on the previous bit, namelythe start state of the modulator. In traditional modulation the controlis based only on one bit (0,1) , that is, usually by turning the heatingof the second waveguide in turn on (1) and off (0). Using two bitsinstead of one and by optimising the fine structures of the rise/falltime periods, the speed can be significantly increased with respect totraditional modulation, but at the same time the power consumptionincreases and some kind of control logic (electronics or a computerprogram) is needed between the bit stream (or similar) and the heatingsignals. By increasing the number of bits that effect a certain riseand/or fall time period above two, it is possible to further acceleratethe operation of a modulator or to reduce the power consumption.

The modulation method according to the invention somewhat increases theelectric power consumption of the component, especially during the riseand fall time periods, and requires somewhat more complicated controlelectronics than conventionally. These are, nevertheless, rather smallproblems compared to the significant increase in the speed. The powerconsumption can also be reduced by reducing the size of the waveguideand by improving thermal insulation (with insulation grooves).

EXAMPLE

The method according to the invention is tested in practise withthermo-optic switches based on silicon-on-insulator (SOI) technology.The operating frequency of the switches has already been raised above160 kHz, while commercial thermo-optical switches usually operate at a 1kHz frequency, at the most.

The extinction ratio of the switches is not perfect due to theirnon-ideal structure (r≠1). Additionally, switches are polarisationdependent because of the thermal oxide used as a cladding. These factorslimit the optical characteristics of the switches, mostly theirextinction ratio, but they have not much of an interaction with the usedmodulation principle. Therefore, the optical characteristics of theswitches can be easily improved without substantially influencing theirspeed.

With traditional modulation methods (cf. FIGS. 3 and 4) the switcheshave been successfully operated with a maximum frequency of approx. 10kHz, whereupon the shape of the pulses already starts to significantlydegrade. By using the modulation method according to the invention, theswitches have been successfully operated at a frequency above 160 kHz,which is at least one (or perhaps even two) orders of magnitude fasterthan comparable commercial thermo-optical switches. The requiredelectrical signals have been obtained with a simple and inexpensiveapparatus.

FIGS. 8 and 9 represent measurement results obtained at frequencies 24,3kHz and 161 kHz, which clearly verify the practical functionality of themodulation principle. At the 161 kHz frequency (FIG. 9) the minimum andmaximum values of the optical signal still depart less than 5% of thoseoptimum values that are obtained with very slow traditional modulation.The amplitudes of the control signals for the waveguides, namely theheating powers, are adjusted so that the optical signal P opt has beenmade to change back and forth as fast as possible. In the exampledepicted in FIG. 8, the switch has been kept either in on- or off-statefor 18 μs between the approx. 2 μs long rise and fall time periods. Inthe example depicted in FIG. 9, the switch has been kept either in on-or off-state for 1 μs between the approx. 2 μs long rise and fall timeperiods.

In FIG. 8, curves P1 and P2 represent changes of the electrical powerconducted to the heating resistors of the waveguides, i.e. heatingpower, for changing the state of the switch between on- and off-states.In the off-state preheating of both waveguides is used, so that theheating powers P1 and P2 are approximately equal. When the switch isguided from the off-state to the on-state, a high power peak is formedinto the curve P2 in the beginning of the rise time period and, at thesame time, the heating power P1 is dropped to zero. The power peak andthe simultaneous dropping of the heating power P1 to zero causes a steepfall in the optical power P opt, i.e. the switch shifts fast from theoff-state to the on-state. In the on-state the heating power P2 isadjusted to a value which is enough to maintain the on-state. The falltime period from the on-state to the off-state, on the other hand,starts with a high power peak of the heating power P1, so that P2 is atthe same time dropped from its on-state power to zero. Then the opticalpower P opt rises steeply, until the off-state is reached. Heatingpowers P1 and P2 are set back to their constant off-state values.

A similar operation can be seen in FIG. 9, where, though, the opticalsignal P opt follows the changes of the heating powers P1 and P2 with adelay, because of the time elapsed during heat transfer and measurementof the optical output.

Heating power signals P1 and P2 of both waveguides, as seen in FIGS. 8and 9, are quite non-ideal when compared with the schematic illustrationin FIG. 5, which is due to the simplicity of the electrical components.The advantages of the modulation principle according to the inventionhave, though, been clearly demonstrated with these very inexpensivecomponents. The total power consumption is mostly determined by the fastpower peaks used to change the state of the switch. Therefore, the lesstime the switch is held in an on- or off-state between the actual riseand fall times, the higher the power consumption is. Average powerconsumptions corresponding to FIGS. 8 and 9 are approx. 0,36 W (24,3kHz) and 0,81 W (161 kHz), respectively.

The invention is not limited to concern the above presented embodimentexamples only, but many variations are possible within the inventionalidea determined by the claims.

1. A method for controlling an optoelectronic component during a risetime period with control signals (1, 2), in which component there are atleast two waveguides optically coupled to each other, the firstwaveguide (3) and the second waveguide (4), which form tracks to anoptical signal, and in the beginning of which rise time period bothcontrol signals (1, 2) are on a common start level, namely on the firstamplitude level (I), which is clearly higher than zero, so that therefractive indices of the waveguides (3, 4) are equal and the phasedifference between them is zero, and at the end of which rise timeperiod the first control signal (1) is on its target level, namely onthe second amplitude level (II), which is clearly higher than the startlevel, and the second control signal (2) is correspondingly on its owntarget level, namely on the third amplitude level (III), which isclearly lower than the start level, so that the refractive indices ofthe waveguides (3, 4) are unequal and there is a predetermined phasedifference between them, and the length of which rise time period islimited by the time required for generating and stabilising a phasedifference change between the waveguides, and in which method the risetime period is shortened by adjusting the control signals between theirstart and target levels in an appropriate manner, characterised in thatfor shortening the rise time period the second control signal (2) isfirst lowered to a fifth amplitude level (V), which is zero orsubstantially lower than the third amplitude level (III), andsimultaneously the first control signal (1) is set to a fourth amplitudelevel (IV), which is clearly higher than the second amplitude level(II), and finally both control signals are set to their target level,and during which rise time period the control signals are adjusted sothat in the last part of the rise time period the phase differencebetween the waveguides is already substantially settled to its targetvalue, while the refractive indices of the individual waveguides arestill clearly changing towards their target values, i.e. settling. 2.Method according to claim 1, characterised in that for obtaining a morestep-wise phase difference change, the first control signal (1) is setfrom the fourth amplitude level (IV) to a seventh amplitude level (VII),which is lower than the second amplitude level (II), before it is set tothe second amplitude level (II).
 3. Method according to claim 2,characterised in that the seventh amplitude level (VII) is chosen to besubstantially equal with the fifth amplitude level (V).
 4. Methodaccording to claim 1, characterised in that for shortening the rise timeperiod, the fourth amplitude level (IV) of the first control signal (1)is chosen so high that the phase difference clearly tends to rise abovethe predetermined target value of the phase difference, so that a socalled overshoot of the refractive index is formed, and for compensatingthe overshoot the amplitude of the second control signal (2) is raisedto a sixth amplitude level (VI), which is higher than the fifthamplitude level (V).
 5. Method according to claim 1, characterised inthat for shortening the rise time period, the target level of the secondcontrol signal (2), namely the third amplitude level (III), is chosen tobe higher than zero.
 6. A method for controlling an optoelectroniccomponent during a fall time period with control signals (1, 2), inwhich component there are at least two waveguides optically coupled toeach other, the first waveguide (3) and the second waveguide (4), whichform tracks to an optical signal, and at the beginning of which falltime period the first control signal (1) is on its start level, namelyon the second amplitude level (II), which is clearly higher than thetarget level, and the second control signal (2) is correspondingly onits own start level, namely on the third amplitude level (III), which isclearly lower than the target level, so that the refractive indices ofthe waveguides (3, 4) are unequal and there is a predetermined phasedifference between them, and in the end of which fall time period bothcontrol signals (1, 2) are on a common target level, namely on the firstamplitude level (I), which is substantially higher than zero, so thatthe refractive indices of the waveguides (3, 4) are equal and the phasedifference between them is zero, and the length of which fall timeperiod is limited by the time required for generating and stabilising aphase difference change between the waveguides, and in which method thefall time period is shortened by adjusting the control signals betweentheir start and target levels in an appropriate manner, characterised inthat for shortening the fall time period the first control signal (1) isfirst lowered to an eighth amplitude level (VIII), which is zero orsubstantially lower than the first amplitude level (I), andsimultaneously the second control signal (2) is set to a ninth amplitudelevel (IX), which is substantially higher than the first amplitude level(I), and finally both control signals are set to the first amplitudelevel (I), and during which fall time period the control signals areadjusted so that in the beginning of the rise time period the refractiveindex difference between the waveguides decreases fast to zero andduring the last part of the fall time period it substantially remains atzero, so that the phase difference between the waveguides is alreadysubstantially settled to zero, while the refractive indices of theindividual waveguides are still clearly changing towards their commontarget value, i.e. settling.
 7. Method according to claim 6,characterised in that for obtaining a more step-wise phase differencechange, the second control signal (1) is set from the ninth amplitudelevel (IX) to the tenth amplitude level (X), which is lower than thefirst amplitude level (I), before it is set to the first amplitude level(I).
 8. Method according to claim 7, characterised in that the tenthamplitude level (X) is chosen to be substantially equal with the eighthamplitude level (VIII).
 9. Method according to claim 1, characterised inthat at least one of the following is chosen to be zero: third amplitudelevel (III), fifth amplitude level (V), seventh amplitude level (VII),eighth amplitude level (VIII), tenth amplitude level (X).
 10. Methodaccording to claim 1, characterised in that the phase difference betweenthe first and the second waveguide is modulated with two or severalsuccessive modulators (10, 11).
 11. Method according to claim 10,characterised in that the modulators (10, 11) are controlled to operatein turn.
 12. Method according to claim 11, characterised in that onlysome of the successively intended phase difference changes areimplemented with control signals corresponding to one modulator (10) andother phase difference changes are implemented with the control signalsof the following one or several modulators that are arrangedsuccessively with respect to the aforementioned modulator (11) so thatthe phase difference changes caused by them sum up, so that the nextphase difference change can be implemented with the next modulator assoon as the previous phase difference change is implemented, althoughthe refractive indices of the waveguides of the modulator thatimplemented it have not yet settled to their target levels.
 13. Methodaccording to claim 1, characterised in that at least two successivemodulators (10, 11) are mutually different so that the first modulator(10) is significantly faster and consumes more power than the secondmodulator (11), so that the first modulator is used for implementingfast and/or successive phase difference changes and the second modulatoris used for implementing slow and/or single phase difference changes andfor implementing long static operating states, so that the average powerconsumption is significantly smaller than by using only the firstmodulator and the maximum modulation speed is significantly higher thanby using only the second modulator.
 14. Method according to claim 1,characterised in that during their rise and/or fall time periods thecontrol signals are optimised so that they depend, not only on the startand target state of the phase difference, but also on at least one phasedifference change that precedes the start state and/or succeeds thetarget state of the phase difference, so that the optimisation takesinto account such potential settling time that immediately precedesand/or succeeds the given phase difference change, and during whichsettling time the phase difference has already reached its target value,but the refractive indices of the waveguides have not yet settled. 15.Method according to claim 1, characterised in that the predeterminedtarget value of the phase difference is set to be approx. 180°. 16.Method according to claim 1, characterised in that the waveguides (3, 4)are arranged as waveguides on planar substrates.
 17. Method according toclaim 16, characterised in that the waveguides (3, 4) are chosen among:SOI (silicon-on-insulator) waveguides, glass waveguides, polymerwaveguides, compound semiconductor waveguides.
 18. Method according toclaim 1, characterised in that the optoelectronic component is chosen tobe an optical switch, like an interferometric optical switch.
 19. Methodaccording to claim 1, characterised in that the optoelectronic componentis chosen to be a component which includes one or several Mach-Zehnderinterferometers, which forms an optical switch or a filter.
 20. Methodaccording to claim 1, characterised in that the optical switch is chosento be a thermo-optic switch where the modulators (10, 11) are heatingelements (5, 6; 7, 8) that heat the waveguides (3, 4) and electricalcontrol signals (1, 2) consist of control voltage/current pulsesdirected to the heating elements, so that a control signal amplitudelevel corresponds to the heating power induced in the heating element bya control voltage/current pulse.